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The galaxy is waiting

The problem with the gravisphere criterion is, obviously, to separate the spheroids from the potatoids. Ceres, Pluto (maybe Charon, the nomenclature isn't quite clear to me at this) and 2003UB313 should be a safe bet. I haven't seen much data about 2005FY9 but from the size estimates I've seen I think it will quickly join together with Orcus. Quaoar, Varuna etc. will need a closer look, and looking closely at Sedna isn't an easy thing to do, for geometric reasons.

2003EL61 is sort of a problem, rotation should turn a sphere into a disc like ellipsoid. EL61 is no classical rotational shape, what would it look like if stopped? Why does it extend mostly along an axis instead its equatorial plane? Is this a hydrostatic equilibrium? I don't know. But I'm sure that this cosmic cigar will be featured in many books.
Growing.

Is Chiron in hydrostatic equilibrium? Its not very big, but is it in that state?

Although, I seemed to recall its perhaps recognised as a dirty big comet these days....

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

*How many do you need?



*quoting the BA from a recent interview.

Personally, I'd like to see the dwarf planet definiton being inclusive in nature - and this means that a body such as Vesta should have a good claim on being included as a dwarf planet in future.

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"All truth passes through three stages. First, it is ridiculed, second it is violently opposed, and third, it is accepted as self-evident." Arthur Schopenhauer - renowned 19th Century German philosopher.

I suspect that the hydrostatic equilbrium issue will result in smaller objects for the KBOs than for asteroids such as Ceres (a dwarf planet) and Vesta (perhaps just a tiny bit too rigid to be a dwarf planet). Object made with a higher concentration of ices might be slump into a spheroid enough shape at about 500 km diameter (my wild guess, not a fact). If so, we will probably have thousands of dwarf planets known before the year 2025, after the GAIA data has been analysed.

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Forming opinions as we speak

How many dwarf planets do we have at the moment? Three - Ceres, Pluto and 'Xena'.

It's not clear what the status of Charon is. If it's considered a satellite of Pluto, then it's not a dwarf planet. If it isn't, then it is. Unfortunately, there is no official definition of satellite, and there will no doubt be a debate on this. Personally, I have some sympathy with a definition based on the barycentre being within the primary (except I'd say 'always within').

There are about a dozen other possibilities, but they'll need further information and examination to be certain, and in the meantime they won't get the classification.

Michael Brown lists 45 on his webpage plus apparently about 30 yet unpublished from his newest survey. He also gives an estimate of 200 for the Kuiper belt.

I believe they stated a Dwarf Planet must attain a "nearly round" shape, because the earth, the moon, and any other fast rotating solid body is not going to be perfectly round.

I classify something as "Nearly Round" if you take a 1 Second Test - you look at a picture of it, and have to say right then and there if its a Circle or Not a Circle. Mimas, Ceres, etc, can all pass for circles in pictures - even if extended study reveals them to not be perfect spheres at all. However, Vesta, Pallas, Hygiea, Davida, the biggest other asteroids, would never pass the 1 Second Test. Vesta looks like a piece of fruit that impacted pavement from 20 feet up. Pallas looks like an egg, and Hygiea and Davida are potatoes.

Just my thoughts. I know a "1 Second Test" isn't exactly scientific ...

---Vil.

So, pareidolia is a scientific criterion?

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

No. The definition states that a dwarf planet must have "sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium", which they then paraphrased in brackets as 'nearly round'.

'Nearly round' is a reasonable first-order approximation - presumably there as an explanation for the journalists and others who might not know what the 'sufficent mass etc' bit meant.

But the criteria is the first bit.

The problem with hydrostatic equilibrium is that you need to analyze the body very carefully. When it comes to KBOs that means we lack detailed observational data to support the hydrostatic equilibrium hypothesis, therefore, we have to assume equilibrium if they have a round shape.

The problem with "roundness" is that we don't have precise limits for it, moreover, it depends on composition and internal temperature.

As a result, we could have more than 50 dwarf planets if we confirm their round shape.

I'd call Charon a dwarf, as the barycenter is always above Pluto's surface.

I think I'd go at that definition from the perspective of it being a moon unless the barycenter is always above the surface of the primary.

Good question! Next question, anyone?
Like KBOs. The centaurs in general are under this suspicion.

It has to be in hydrostatic equilibrium due to its self-gravity, and given its dimensions of 132-142 km, it's too small (unless it's made of degenerate matter or something!). Chiron wasn't on the list of possible candidate 'planets' that the IAU attached to their draft definition, which it would have been if anyone thought there was a chance it was in hydrostatic equilibrium due to its self-gravity.

I don't think so! Unless it's big enough like 'Xena' for it's self-gravity to certainly mean it is in hydrostatic equilibrium, the mere fact of roundness will not be sufficient to confirm that. Things can be round for other reasons.

Don't get hung up on roundness. That's not the criteria.

I should imagine we will have a long list of candidate dwarf planets that it may take some time to decide on whether they meet the criteria or not. But if it takes time, it takes time. There's no rush!

This paragraph seems to contradict itself, or does it?

Could you rephrase it?

Doesn't seem contradictory to me. He stated that, unless its composed of exotic matter, its too small (not massive enough) to be subject to hydrostatic equillibrium. Then he says that he doesn't think the IAU believes it either, as it wasn't on the list of potential candidates in the earlier draft of the definition.

Did I miss something?

No, I did. I misread (I think!) the first part of the first sentence "It has to be in hydrostatic equilibrium due to its self-gravity" as a claim about Chiron, rather than a statement of the requirements of the (previously proposed) IAU rule.

It occurs to me that the biggest problem in determining hydrostatic equilibrium is that we do not know the density of most of the KBOs. We know the strength of molecular bonds and van der Wal forces in the molecules that could make up the KBOs, but we don't have a decent profile for any of the KBOs except for Pluto and Charon. For most of them, I'm not sure how we'd be able to obtain such a profile short of sending a probe to each and every one of them.

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